Understanding the SDM

ML-Series cards use the forwarding engine and ternary content-addressable memory (TCAM) to implement high-speed forwarding. The high-speed forwarding information is maintained in TCAM. The SDM is the software subsystem that manages the switching information maintained in TCAM.

SDM organizes the switching information in TCAM into application-specific regions and configures the size of these application regions. SDM enables exact-match and longest-match address searches, which result in high-speed forwarding. SDM manages TCAM space by partitioning application-specific switching information into multiple regions.

TCAM identifies a location index associated with each packet forwarded and conveys it to the forwarding engine. The forwarding engine uses this location index to derive information associated with each forwarded packet.

The key benefits of SDM in switching are its ability to organize the switching information in TCAM into application-specific regions and its ability to configure the size of these application regions. SDM enables exact-match and longest-match address searches, which result in high-speed forwarding.

SDM Regions

SDM partitions TCAM space into multiple application-specific regions and interacts with the individual application control layers to store switching information. SDM consists of the following types of regions:

•Longest-match region—Each longest-match region consists of multiple buckets or groups of Layer 3 address entries organized in decreasing order by mask length. All entries within a bucket share the same mask value and key size. The buckets can change their size dynamically by borrowing address entries from neighboring buckets. Although the size of the whole application region is fixed, you can reconfigure it.

•Weighted-exact-match region—The weighted-exact-match region consists of exact-match-entries with an assigned weight or priority. For example, with QoS, multiple exact match entries may exist, but some have priority over others. The weight is used to select one entry when multiple entries match.

TCAM space consists of 65,536 entries, each entry being 64 bits wide. Because SDM is responsible for managing TCAM space, SDM partitions the entire TCAM space for each application region based on user configuration. Although the maximum size of all application regions is fixed, you can reconfigure the maximum size of each application region.

Configuring SDM

This section describes the commands necessary to configure the SDM. It includes commands to configure the size of the SDM regions. The commands described in this section are unique to the switching software.

Configuring SDM Regions

TCAM space consists of 65,536 entries, each entry being 64 bits wide. Since SDM is responsible for managing TCAM space, SDM partitions the entire TCAM space for each application region based on user configuration. A change in the partition configuration takes effect the next time you reboot the system.

The application region size in SDM is represented by the number of 64-bit entries. The combined size of all the application regions should be calculated in terms of 64-bit TCAM entries and should not exceed 65,536 bytes, which is the total TCAM size.

To configure SDM maximum size for each application region, perform the following procedure, beginning in global configuration mode:

Command

Purpose

Step 1

Router(config)# sdm sizeregion-name
[k-entries]number-of-entries

Sets the name of the application region whose size you want to configure. You can enter the size as multiples of 1K (that is, 1024) entries or in absolute number of entries.

Step 2

Router(config)# end

Exits to privileged EXEC mode.

Example 14-1 Limiting the IP-Prefix Region to 2K Entries

Router # configure terminal

Router(config)# sdm size ip-prefix k-entries 2

Router(config)# end

To display the number of available TCAM entries, enter the show sdm size command from global configuration mode:

Router # show sdm size

Active Switching Database Region Maximum Sizes :

IP Adjacency : 65536 64-bit entries

IP Prefix : 2048 64-bit entries

QoS Classifiers : 65536 64-bit entries

IP VRF Prefix : 65536 64-bit entries

IP Multicast : 65536 64-bit entries

MAC Addr : 65536 64-bit entries

Access List : 65536 64-bit entries

Configuring Access Control List Size in TCAM

The default maximum size of the Access Control List (ACL) is 65,536 64-bit entries. You can enter the sdm access-list command to limit the TCAM space for ACLs, as shown in Table 14-2.

Table 14-2 Partitioning the TCAM Size for ACLs

Task

Command

sdm access-listnumber-of-entries

Sets the name of the application region for which you want to configure the size. You can enter the size as an absolute number of entries.